The physical phenomenon of light occurs over a continuous range of wavelengths, some of which are visible to the human eye. Color is actually the distribution of energy throughout the visible spectrum of light. To accurately and fully represent the actual energy distribution (color) at a single point, it would require a long list of numbers representing the intensity of the light at various wavelengths.
The human eye doesn't sense color in this level of detail though. The human retina contains three types of color sensitive receptors (cones). Each type of cone has a different range of wavelengths of light for which it is sensitive. The combination of the three different sensitivities divides the visible spectrum into three overlapping sub-ranges that the eye/brain perceives collectively as color. In order for-two colors to look identical, they don't necessarily have to have identical wavelength spectrums. They only need to have the same amount of energy in the three sensitivity ranges that the eye senses.
Most electronic representations of color take advantage of the eye and brain's simplified perception mechanism, and only utilize three numeric values to represent a specific color. There are a variety of different three channel (three numeric values) color spaces that are commonly used. The different color spaces are all representations of the same original spectrum of light in the image. Each channel of a color space represents the weighted energy content over a specific range of wavelengths of light. Each color space is defined by (either directly or indirectly) a set of weighting curves that specify how the continuous color spectrum contributes to each channel of the color space.
Typically, color image sensors in a scanner deliver three channels of data to the image processing electronics, representing the red, green, and blue content of the image. Similar to the eye, these are three overlapping sub-ranges of the visible spectrum of light. However, they are different sub ranges from those of the human eye. They are also generally subtly different from the red, green, and blue of the industry standard color space called sRGB.
The role of the color correction and conversion hardware/software algorithms in image processing electronics is to perform the conversion from the image sensors RGB to standard sRGB, and possibly to another color space. This is commonly performed with a look up table (LUT). Each input vector (sensor RGB combination) maps to a single specific output vector in the output color space. To exhaustively tabulate all possible input colors would require an excessively large look up table, so actual designs usually contain a subset of points, and interpolate between them. This is commonly referred to as a three dimensional (3D) LUT because there are three components in the input vector (image sensor RGB). There are usually three components in the output vector as well, which really makes the system three independent 3D-LUTs, each with a single channel of output.
The accuracy of such a system can only be as good as the contents of the LUT. Most current systems do not vary the contents of the 3D LUT during operation. Instead, a LUT is selected at the time the system is designed. Usually some form of white balance system is employed to ensure that the scaling of the inputs to the 3D-LUT is consistent over time.
Current state of the art white balance subsystems, in similar applications, involve the use of a white reference area introduced into or included in the primary scanning area. The color detected by the main scanning apparatus in the reference area is measured and compared with a predetermined aim point, and corresponding adjustments are made to gain stages in each of the detection system channels.
Although the currently used and utilized system for compensating LUTs is satisfactory, they include drawbacks. These white balance systems compensate for variations in illumination intensity and optical throughput. They do not, however, compensate for all possible variations in the color spectral content of the illumination.
Consequently, a need exists for scanning systems that compensate for the color spectral content of the illumination.